For efficient use of wood resources, it is important to improve the yield of chemical pulp. For one of the high-yielding techniques of kraft pulp, which has become the mainstream of chemical pulp, there is known a polysulfide cooking process. Polysulfide oxidizes the carbonyl end group of carbohydrates to suppress the decomposition of the carbohydrates ascribed to a peeling reaction, thereby contributing to an improved yield. The chemical cooking liquor in the polysulfide cooking process is produced by oxidizing an alkaline aqueous solution containing sodium hydroxide and sodium sulfide, so-called white liquor, with molecular oxygen, such as in air, in the presence of a catalyst such as activated carbon or the like, e.g. by the following reaction formula (1), (Japanese Laid-open Patent Application No. S61-259754 and Japanese Laid-open Patent Application No. S53-92981).
According to this method, there can be obtained a polysulfide cooking liquor having a polysulfide concentration of about 5 g/L at a conversion rate of about 60% at a selectivity of about 60% on the sulfide ion basis. However, in a case where the conversion rate is raised according to this method, thiosulfate ions that do not contribute to cooking at all are secondarily produced in large amounts by side reactions, e.g. by the following formulas (2), (3), so that a difficulty has been involved in the production of a cooking liquor containing a high concentration of polysulfide sulfur at a high selectivity.4Na2S+O2+2H2O→2Na2S2+4NaOH  (1)2Na2S+2O2+H2O→Na2S2O3+2NaOH  (2)2Na2S2+3O2→2Na2S2O3  (3)
On the other hand, in WO No. 95/000701 and WO No. 97/000071, there is described an electrolytic production method of an alkaline cooking liquor containing polysulfide. This method enables an alkaline cooking liquor containing a high concentration of polysulfide sulfur to be produced at a high selectivity while pronouncedly reducing secondary production of thiosulfate ions. Besides, for a method of obtaining an alkaline cooking liquor containing a high concentration of polysulfide sulfur, there is disclosed, in Japanese Laid-open Patent Application H8-311790, a method wherein molecular sulfur is dissolved in an alkaline aqueous solution containing sodium hydroxide and sodium sulfide.
Meanwhile, in order to re-use chemicals after recovery of a cooking spent liquor discharged in the production process of chemical pulp, an important issue is such that a recovery boiler has enough capacity to recover. For a factor of an increased load of the recovery boiler, there are those concerning organic matters and those concerning inorganic matters. The load of the recovery boiler may be mitigated by improving pulp yield for the former and by reducing specific chemical consumption for the latter. Although an available capacity of a recovery boiler is ensured by re-equipping or output reduction, other methods have been demanded from the standpoint of efficiency and cost.
For a saving method of specific chemical consumption, there have been used cooking methods wherein a quinone compound, i.e. a cyclic keto compound, such as an anthraquinonesulfonate, anthraquinone, tetrahydroanthraquinone or the like, is added to a cooking system as a cooking aid (e.g. in Japanese Patent Publication No. S55-1398, Japanese Patent Publication No. S57-19239, Japanese Patent Publication No. S53-45404 and Japanese Laid-open Patent Application No. S52-37803). Quinone compounds contribute to improving delignification selectivity, reducing the Kappa number of cooked pulp, or saving chemicals, and improving the pulp yield. In Japanese Laid-open Patent H7-189153, there is disclosed a cooking process using, in combination, a quinone compound and an alkaline cooking liquor containing polysulfide, and in Japanese Laid-open Patent Application No. S57-29690, there is disclosed moderated decomposition of polysulfide with a quinone compound under heated alkaline conditions.
By the way, a technology of “leveling” of an alkali shift has been introduced according to the pioneer work, Svensk Paperstindning, 87(10): 30 (1984), made by the Swedish STFI Institute from the end of 1970's to the early 1980's. This method, which is characterized by “split addition of white liquor” and countercurrent processing, is known as “modified kraft cooking” and has been widely adopted in the field of pulp industry in 1980's. For instance, this method and its related equipment have been sold under the trademark of MCC. Later, this countercurrent method has been extended to the addition of white liquor to a countercurrent washing zone, known as high-heat washing zone”, and commercially sold under the trademark of EMCC.
Furthermore, in 1990's, the Lo-Solids (registered trademark) cooking process and its related equipment have been introduced and have become subsequent drastic improvements of the kraft cooking process (U.S. Pat. Nos. 5,489,363, 5,536,366, 5,547,012, 5,575,890, 5,620,562 and 5,662,775). In this process, strong and pure cellulose pulp can be made by selectively withdrawing a spent cooking liquor at an initial stage of the pulp manufacturing process and supplementing a cooking liquor and a dilute liquor, e.g. a washer filtrate containing only a low concentration of dissolved matters.
In Japanese Laid-open Patent Application Nos. 2000-336586 and 2000-336587, there have been proposed techniques of improving pulp yield in association with such a novel cooking process. These proposals provide a cooking process of a lignocellulose material, characterized by making use of hardwood or softwood chips, adding, at a top of the digester, an alkaline cooking liquor that contains polysulfide sulfur at a sulfur concentration of 3˜20 g/L and further contains 45-100 mass % of a sulfur component relative to a sulfur component of total cooking activity and contains 45-79 mass % of effective alkali relative to total alkali, respectively, contained in an alkali cooking liquor to be introduced into a digestion system, and further feeding an alkaline cooking liquor containing 0.01˜5 mass % of a quinone compound based on bone-dry chip to the digester.
However, there has been a demand of further improving the pulp yield or reducing the specific chemical consumption.